This invention relates to a glass article which is made of an ordinary oxide glass represented by soda-lime-silicate glass and is afforded with high mechnaical strength by an ion exchange treatment of its surfaces and to a method of producing the same.
A well known method of increasing mechanical strength of a glass article by a chemical treatment is the so-called low-temperature type ion exchange method. This method comprises contacting the glass article with a source of an alkali metal ion relatively large in ionic radius, such as potassium ion, to cause at least partial exchange of a relatively small alkali metal ion, such as sodium ion, in the surface layers of the glass article with the larger alkali metal ion. The ion exchanging treatment induces compressive stresses in the surface layers of the glass article. At present the low-temperature type ion exchange method is prevailing for strengthing relatively thin glass sheets and relative small-sized or intricately shaped glass articles.
In principle, by the ion exhcange method it is possible to induce greater compressive stresses in the glass surfaces than by the thermal tempering method. In practice, however, the ion exchange method tends to entail relatively high costs. Basically the ion exchanging treatment requires a considerably long time, and the efficiency of the treatment and the degree of strengthing depend greatly on the composition of the glass. Therefore, it is often to use a specially formulated glass composition containing a considerably increased amount of Na.sub.2 O or Li.sub.2 O or containing relatively large amounts of auxiliary components such as, for example, Al.sub.2 O.sub.3 and/or ZrO.sub.2. Such modification of the glass composition leads to an increase in material cost and, besides, is liable to unfavorably affect some properties of the glass as represented by a decrease in the weather resistance of the obtained glass or an increase in the tendency toward devitrification during the glass manufacturing process.
U.S. Pat. No. 3,798,013 shows a method of strengthening a glass article by two-stage ion exchange treatment of the low-temperature type. In the first-stage treatment of a glass containing sodium ion as the principal alkali metal ion, a mixture of a sodium salt and a potassium salt is used as the external source of alkali metal ions. In the second-stage treatment a potassium salt alone is used as the external source of alkali metal ion, and the treating temperature is lower than in the first-stage treatment and/or the treatment time is shorter than in the first-stage treatment. This method is proposed with a view to shortening the total time of ion exchange treatment for accomplishment of sufficient strengthening of the glass article. However, the mechanical strength of the obtained glass article is not very higher than that of a similar glass article strengthened by the conventional single-stage ion exchange treatment.